Process for preparing perfluoroalkanedioic acids



United States Patent PROCESS FOR PREPARING PERFLUORGALKANE- DIOIC ACIDSJohn J. Drysdale, Wilmington, Del., assignor to E. I. (11! Pont deNemours and Company, Wilmington, Del., a

corporation of Delaware No Drawing. Filed May 15, 1961, Ser. No. 109,822

7 Claims. (Cl. 260--537) This invention relates to a process forpreparing perfluoroalkanedioic acids and more particularly for preparingsuch acids by the hydrolysis of certain bis(alkylthio)-perfluoroalkanes.

Perfiuorodicarboxylic acids of structure are a valuable series of knowncompounds. Those, wherein m is greater than one, are known to be usefulintermediates for the preparation of polyester lubricants and the likewhich are highly stable thermally and oxidatively (OBrian et al.,W.A.D.C., Tech. Rept. No. 56-11, A.D. 110674). Several methods are knownfor preparing perfluorodicarboxylic acids but they are, in general,rather poor, requiring starting materials which are not readily obtainedor involving reactions which give poor yields at best. It is an objectof this invention to provide an improved process for preparingperfluoroalkanedio-ic acids. Another object is to provide a process forpreparing perfluoroalkanedioic acids in high yields from readilyavailable starting materials. A further object is to provide a processwhich is simple and readily carried out and produces theperfluoroalkanedioic acids in a more economical manner. Another objectis to advance the art. Still other objects will appear hereinafter.

The above and other objects may be accomplished in accordance With thisinvention which comprises heating a bis(alkylthio)perfluoroalkane of theformula wherein R is an alkyl radical of 1 to 6 carbon atoms and n is aninteger of from 2 to 20 with from about 1 to about 4 parts by weight ofconcentrated sulfuric acid of from about 80% to 100% strength at atemperature bis(alkylthio)perfiuoroalkanes are readily availablematerials known to the art. The process is easy to operate and controland economically attractive.

The bis(alkylthio)perfluoroalkanes which may be employed .in the processof this invention have the formula 'RS(CF CF ),,SR wherein R is an alkylradical of l to '6 carbon atoms and n is an integer of from 2 to 20.While R may be almost any saturated alkyl radical, it is desired(primarily for economical reasons) that the alkyl radicals contain notmore than 6 carbon atoms, and preferably methyl. employed in thisinvention conveniently are prepared by the method described by Hanfordin US. Patent 2,443,- 003, and particularly in Example 2 thereof. Bysuch process, there is produced a series of compounds of the formulaRS(CF CF ),,SR wherein n has a series of in- The bis alkylthioperfiuoroalkanes tegral values of 1 to 25 or perhaps greater.Representative bis(alkylthio)perfluoroalkanes which are suitable for thepurpose of this invention are: l,4-bis(methylthio)- perfluorobut ane (CHS(CF SCH 1,6 bis(methylthio)perfiuorohexane (CH S(CF SCH1,8-bis(methylthio)perfiuorooctane (CH S(CF SCH 1,10 bis-(methylthio)perfluorodecane (CH S(CF SCH 1,12- bis(methylthio)perfluorododecane (Cl-I S(CF SCH l,14-bis(methylthio)perfluorotetradecane 1,15bis(methylthio)perfluorohexadecane and1,18-bis(methylthio)perfluorooctadecane.

The sulfuric acid employed should have a concentration of from about to100% strength. Usually, the commercial concentrated sulfuric acid ofabout to about 98% strength will be employed. Also, the sulfuric acidshould be employed in the proportion of from about 1 to about 4 parts byweight per part of the bis(alkylthio perfluoroalkane.

The temperature for carrying out the reaction may be from about 100 C.to about 350 C., and preferably from'about C. to about C. The reactiontime usually will vary from about 1 to about 10 hours, depending uponthe particular starting material, the temperature employed, andeconomical factors such as the cost of more extended periods of heatingand the increase in yield to be obtained thereby.

A number of methods are available for recovering the product from thereaction mixture. One method, which was employed in the examplespresented hereinafter and which is suitable for those acids which-arereasonably soluble in dilute aqueous sulfuric acid, is to add the cooledreaction mixture to cooled water, filter the resulting aqueous solution,and then extract the dicarboxylic acid from the aqueous sulfuric acidwith a suitable solvent. The perfiuoroalkanedioic acids which contain upto about 12 carbon atoms are reasonably soluble in water and in diluteaqueous sulfuric acid solutions, but can be crystallized from theaqueous solutions as the dihydrates. During the reaction, certainamounts of sulfur and tarry material are formed which are insoluble inthe dilute aqueous solution, and such materials and unreacted startingmaterial are removed by filtration of the cold dilute sulfuric acidsolution. The perfiuoroalkanedioic acid will be extracted from theaqueous solution by a lower dialkyl ether such as diethyl ether,dipropyl ether and dibutyl ether. The extracted solvent is thenevaporated to leave a residue of the perfluoroalkanedioic acid. Suchperfiuoroalkanedioic acid sometimes will contain a small amount ofpartially hydrolyzed material having the formula RS(CF CF ),,CO H whichis Water soluble and is extracted with the ether. If necessary, theperfiuorodialkanedioic acid may be purified by recrystallization,distillation, or the like.

When the perfiuoroalk anedioic acids are insufficiently soluble indilute aqueous sulfuric acid, i.e. contain more than 12 carbon atoms,the cooled reaction mixture can be added to cooled water and theresulting mixture made basic with alkali, such as sodium hydroxide,sodium carbonate, and the like, to form the water-soluble alkali salts.After filtering the salt solutions to remove sulfur,

tarry material and unreacted starting material, the solution can bereacidified to precipitate the periluoroalkanedioic acid. In this casealso, partially hydrolyzed material (if present) will be carried intothe perfluoroalkanedioic acid, which can be further purified byrecrystallization, distillation, etc.

Representative acids which may be prepared by the present processinclude perfluorosuccinic acid perfiuoroadipic acid (HO C(CF CO H),perfluorosuberic acid (HO C(CF CO H), perfluorosebacic acid (HO C(CF COH) perfluorododecanedioic acid (HO C(CF CO H), perfiuorotetradecanedioicacid (HO C(CF CO H), and perfluorooctadecanedioic acid (HO C(CF CO H).

In order to more clearly illustrate this invention, preferred modes ofcarrying it into effect, and the advantageous results to be obtainedthereby, the following examples are given in which the parts andproportions are by weight except where indicated otherwise.

Example 1 1,6-bis(methylthio)perfluorohexane (29.4 parts) was heatedwith 40 parts of concentrated (95-98%) sulfuric acid at 150 C. for onehour. The reaction mixture was then cooled and added to ice-water. Theaqueous phase was filtered to remove sulfur and tarry material, and thefiltrate extracted with diethyl ether. The ether extracted with diethylether. The ether extract, on drying and evaporation, gave parts ofperfiuoroadipic acid as di hydrate (62% yield). The acid was identifiedby converting it to several known derivatives such as the dianiliniumsalt, the dimethylester, the diethylester, and the diacid chloride.

Example 2 1,4-bis(methylthio)perfluorobutane (29.4 parts) was heatedwith parts of concentrated (95-98%) sulfuric acid at 165 C. for twohours. The perfluo'rosuecinic acid as the dihydrate (16.8 parts, 74%yield) was isolated as in Example 1. The acid was identified via itsknown di anilinium salt and dimethyl ester.

Example 3 l,8-bis(methylthio)perfluorooctane (39.4 parts) was heatedwith stirring with 40 parts of concentrated (95 98%) sulfuric acid forone hour at 155 C. to 160 C. The perfluorosuberic acid as the dihydrate(23.4 parts, 69% yield) was isolated as in Example 1, and identified viaits dianilinium salt, M.P. 183 C., and the diacid chloride, B.P. 112C./l mm.

Example 4 l,6-bis(methylthio)perfluorohexane (29.4 parts) was heatedwith 91 parts of concentrated (95-98%) sulfuric acid for six hours at165 C. The perfluoroadipic acid as the dihydrate (19.4 parts, 78% yield)was isolated as in Example 1.

In place of the bis(methylthio)perfluoroalkanes used in the aboveexamples, the bis(ethylthio), bis(propylthio), bis(butylthio), and thelike perfluoroalkanes can be used with equal facility.

An alternate method of isolation of the perfluoroalkanedioic acids hasbeen found which avoids the extraction with the highly flammable ethersas disclosed in the foregoing examples. In this alternate method, thereaction mixture is added to a cold mixture of water and a loweralcohol, such as methanol, ethanol, propanol and isopropanol, preferablymethanol, preferably employing about a fourfold excess of alcohol overthat theoretically required to convert the perfiuoroalkanedioic acid tothe diester. Thereby, the perfiuoroalkanedioic acids are converted totheir diesters, which are materially more soluble in the aqueous alcoholsolutions. The cold mixture is then filtered to remove the insolubleby-products and the filtrate heated to about 60 C. and continuouslyextracted with an immiscible solvent in a continuous extractionapparatus. Suitable immiscible solvents are: 1,2-dichloroethane,chloroform, methylene dichloride, trichlorotrifiuoroethane, and dibutylether. The diester is collected by evaporation of the solvent, andseparated from any partially hydrolyzed product by distillation of theresidual ester. Using this method and the reaction procedure of Example2, a 67% yield of dimethyl perfiuorosuccinate, B.P. 1l2-113 C./100 mm.,is obtained. Other esters isolated in this manner are dimethylperfluoroadipate, B.P. l20-121 C./50 mm., and diethyl perfiuoroadipate,B.P. 101 C./ 10 mm. The esters are then hydrolyzed, using dilute aqueousacid, to the acids which are collected by evaporation of the aqueoussolution. For many purposes, the esters may be used in place of theacids for preparing useful derivatives.

It will be understood that the preceding examples have been given forillustrative purposes solely and that this invention is not limited tothe specific embodiments described therein. On the other hand, it willbe readily apparent to those skilled in the art that, subject to thelimitations set forth in the general description, many variations can bemade in the materials, proportions, and conditions employed withoutdeparting from the spirit or scope of this invention.

From the preceding description, it will be apparent that the presentinvention provides a new and improved process for preparingperfiuoroalkanedioic acids readily and economically in a single reactionstep from readily available starting materials. Accordingly, it will beapparent that this invention constitutes a valuable advance in andcontribution to the art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. The process for preparing a perfiuoroalkanedioic acid which comprisesheating a bis(alkylthio)perfluoroalkane of the formula RS(CF CF ),,SRwherein R is an alkyl radical of 1 to 6 carbon atoms and n is an integerof from 2 to 20 with from about 1 to about 4 parts by weight ofconcentrated sulfuric acid of from about to strength at a temperature offrom about 100 C. to about 350 C., and recovering theperfluoroalkanedioic acid from the reaction mixture.

2. The process for preparing a perfiuoroalkanedioic acid which comprisesheating a bis(alkylthio)perfluoroalkane of the formula RS(CF CF ),,SRwherein R is an alkyl radical of 1 to 6 carbon atoms and n is an integerof from 2 to 20 with from about 1 to about 4 parts by weight ofconcentrated sulfuric acid of from about 80% to 100% strength at atemperature of from about C. to about C., and recovering theperfiuoroalltanedioic acid from the reaction mixture.

3. The process for preparing a perfiuoroalkanedioic acid which comprisesheating a bis(alkylthio)perfluoroalkane of the formula RS(CF CF SRwherein R is an alkyl radical of 1 to 6 carbon atoms and n is an integerof from 2 to 20 with from about 1 to about 4 parts by weight ofconcentrated sulfuric acid of from about 95% to about 98% strength at atemperature of from about 150 C. to about 175 C., and recovering theperfluoroalkanedioic acid from the reaction mixture.

4. The process for preparing a perfiuoroalkanedioic acid which comprisesheating a bis(alkylthio)perfluoro alkane of the formula RS(CF CF SRwherein R is an alkyl radical of l to 6 carbon atoms and n is an integerof from 2 to 6 with from about 1 to about 4 parts by weight ofconcentrated sulfuric acid of from about 95% to about 98% strength at atemperature of from about 150 C. to about 175 C., and recovering theperfiuoroalkanedioic acid from the reaction mixture.

5. The process for preparing perfiuoroadipic acid which comprisesheating a bis(alkylthio)perfluorohexane 5 of the formula RS(CF CF SRwherein R is an alkyl radical of l to 6 carbon atoms with from 1 toabout 4 parts by weight of concentrated sulfuric acid of from about 95%to about 98% strength at a temperature of from about 150 C. to about 175C., and recovering perfluoroadipic acid from the reaction mixture.

6. The process for preparing perfluorosuccinic acid which comprisesheating a bis(alkylthio)perfiuorobutane of the formula RS(CF CF SRwherein R is an alkyl radical of 1 to 6 carbon atoms with from 1 toabout 4 parts by Weight of concentrated sulfuric acid of from about 95%to about 98% strength at a temperature of from about 150 C. to about 175C., and recovering perfluorosuccinic acid from the reaction mixture.

7. The process for preparing perfluorosuberic acid which comprisesheating a bis(alkylthio)perfiuorooctane of the formula RS(CF CF SRwherein R is an alkyl radical of 1 to 6 carbon atoms with from 1 toabout 4 parts by weight of concentrated sulfuric acid of from about 95%to about 98% strength at a temperature of from about 150 C. to about 175C., and recovering 10 perfluorosuberic acid from the reaction mixture.

No references cited.

1. THE PROCESS FOR PREPARING A PERFLUOROALKANEDIOIC ACID WHICH COMPRISESHEATING A BIS(ALKYLTHIO)PERFLUOROALKANE OF THE FORMULA RS(CF2CF2)NSRWHEREIN R IS AN ALKYL RADICAL OF 1 TO 6 CARBON ATOMS AND N IS AN INTEGEROF FROM 2 TO 20 WITH FROM ABOUT 1 TO ABOUT 4 PARTS BY WEIGHT OFCONCENTRATED SULFURIC ACID OF FROM ABOUT 80% TO 100% STRENGTH AT ATEMPERATURE OF FROM ABOUT 100* C. TO ABOUT 350*C., AND RECOVERING THEPERFLUOROALKANEDIOIC ACID FROM THE REACTION MIXTURE.